Subduction Zone StudiesWilliam P. Leeman
Keith-Wiess Geological Laboratories
Why Study Subduction Zones?
Introduction to Volcanic Arc systems highlights the importance of understanding how subduction zones operate. For example, many of the Earth's most active 'Decade Volcanoes' are associated with subduction zones. In addition, siesmicity associated with subduction zones may result in significant earthquake and tsunami hazards in contiguous areas. Further rationales for scientific investigation of subduction processes are provided in the following links:
Related workshop reports
Lithospheric plates. The uppermost part of the Earth is subdivided into a small number of rigid plates which comprise about 85% of the surface. In places these are separated by non-rigid (deforming) zones. Elsewhere plate boundaries of three types exist: divergent or spreading (e.g., mid-oceanic ridges), convergent (e.g., subduction zones), and strike-slip (e.g., the San Andreas fault zone in California or oceanic transform faults).
A model (NUVEL-1) describing relative plate motions can be used to calculate velocities and azimuths of plate interactions at specific boundaries.
Volcanism commonly is associated with the first two types of margins. An example of the first type is the currently active Axial Seamount which lies on the Juan de Fuca Ridge off the coast of Washington. Convergent margin volcanism produces the 'ring of fire' around the Pacific ocean, and is typified by the Cascade volcanic arc in the Pacific Northwest US. This web page focuses primarily on convergent margins.
Topographic relief maps - surficial expression of convergence
Background information. Use of satellite altimetry to predict global seafloor topography provides unprecedented morphological detail of the seafloor. Such maps reveal the volcanic mid-ocean ridges (e.g., Atlantic) where oceanic plates spread apart, linear volcanic ridges formed by passage of migrating plates over relatively fixed volcanic hot spots (e.g., Hawaii in the mid-Pacific and Reunion in the Indian ocean), and deep troughs or trenches associated with subduction (downward faulting) of oceanic plates beneath overriding continental margin or intra-oceanic volcanic arcs. Spreading ridges are evident behind certain volcanic arcs (e.g., Philippine Sea). Conspicuous mountain belts are associated with collisions between two continental plates (e.g., Himalayas and Alps). Incipient oceanic spreading is evident in the Red Sea and Gulf of Aden. These and many other features are evident in a global color relief map (Mercator projection).
A selection of generic global relief maps (various perspectives) is available for view from NOAA's National Geophysical Data Center (NGDC). The following examples may be particularly interesting:
(clickable for detailed views)
World earthquakes, plate boundaries, and color relief
Western hemisphere earthquakes, plate boundaries, and color relief
Pacific Ring of Fire
Antarctica & South Sandwich Arc
Asian plate boundaries
Africa & Atlantic Ocean
Detailed maps. For more detailed views of specific convergent margins, maps based on the satellite altimetry data have been prepared using the General Mapping Tool (GMT) software. The following arcs can be viewed:
South Sandwich (intermediate detail)
South Sandwich (detailed)
Seismicity at convergent margins
Map views. Convergent margins are among the world's most seismogenic zones, and are characterized by progressively deeper earthquakes as one proceeds from trench to back-arc region - at most convergent margins, these earthquake foci define a dipping plane (the Wadati-Benioff zone, or WBZ) which corresponds to a fault zone between subducting oceanic lithosphere and the overriding plate. A global map of world seismicity shows this relationship. Such zones are characterized by chains of trench-parallel volcanoes (volcanic arcs) fed by magmas rising from depths of up to ~100 km. Regional maps of seismicity (from The USGS National Earthquake Information Center provides regional maps of seismicity for specific volcanic arcs. Examples are linked below:
- note the paucity of deep earthquakes for the Cascadia arc
Two-dimensional cross sections. Cross-arc depth profiles of earthquake foci provide interesting two-dimensional views of the structure of WBZs and display a variety of geometries for subduction zones. The following images were provided by Robert Engdahl (USGS) as projections of WWSN earthquake foci onto a common plane perpendicular to the respective convergent margins.
Watch this space!
Seismic tomographic images. The velocity structures of convergent margins have been imaged using the technique of seismic tomography - wherein arrivals of numerous seismic waves penetrating a study area are analyzed to determine relative velocity variations (anisotropy) in 3-D. Subducting oceanic lithosphere slabs (also defined by earthquake seismicity) are characterized by fast velocities, whereas portions of the upper plate (especially beneath volcanic arcs) have slow velocities that are attributed to ingress of slab-derived fluids, zones of partial melting, or magmatic conduits and reservoirs. Examples of such imagery are available for the back-arc Lau Basin, associated with the Tonga-Kermadec volcanic arc.
Global tomographic inversions for a regionalized Earth model (RUM model) have produced 3-D images of several subducted slabs; global maps are also available showing the data sets on which these inversions are based.
Volcanic arc vital statistics - (watch this space)
Many thanks to Jim Lemaux and Saijin Huang, who provided crucial assistance in preparing many of the graphics files for this web page.
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Updated: 15 Feb 99